1. Field of the Invention
The present invention relates to a polymer electrolyte and a lithium secondary battery including the same, and more particularly, to a polymer electrolyte for a lithium secondary battery that does not dissolve at high temperature and can stably exist in the battery, and a lithium secondary battery including the same.
2. Description of the Related Art
According to the need for smaller, lighter, and more functional portable electric devices, lithium secondary batteries need to become thinner. Recently, thinner lithium secondary batteries have been produced by substituting a polymer electrolyte for a conventional organic electrolyte. Polymer electrolytes are classified into a physical gel type and a chemical gel type according to a preparation method thereof.
The physical gel type is manufactured in the following operations: making a paste by dissolving a polymer by adding an organic electrolyte thereto and heating the polymer organic electrolyte, fabricating a battery by spraying the paste on a sheet positive electrode and, at the same time, loading the negative electrode on the paste, inserting the battery into a battery container and sealing it, and cooling the paste to allow it to become the polymer electrolyte physical gel. A material that is easily gelated by the organic electrolyte and easily dissolves at 80 to 100° C. in the organic electrolyte is preferable as the polymer.
On the other hand, the chemical gel type is manufactured by the following operations: fabricating the electrode assembly by stacking a sheet positive electrode, a non-woven fabric, and a negative electrode, and inserting the electrode assembly, an organic electrolyte, a polymerization initiator, and a monomer such as one from a vinyl group into the battery container. The chemical gel polymer is fabricated by reacting the polymerization initiator with the monomer, with the polymerization of the monomer taking place in the battery container. It is generally preferable that the monomer produces a polymer that can easily gelate by reacting with the organic electrolyte.
The physical and chemical gel types can each be used as the electrolyte of a lithium secondary battery because both have ion conductivity of 2 to 3 mS/cm. However, a lithium secondary battery comprising the physical gel has a problem in that, if the temperature of the battery increases, the polymer that constitutes the physical gel dissolves in the organic electrolyte and is apt to be liquidized. This causes the positive and negative electrodes to short circuit.
On the other hand, the lithium secondary battery comprising the chemical gel has problems in that any polymerization initiator that does not react has a negative influence on the charge and discharge reactions of the battery. Further, gas is produced as a result of the reaction of the polymerization initiator. In addition, the polymer electrolyte does not form because of insufficient polymerization of the monomer.
Also, the negative electrode of the lithium secondary battery is apt to decompose the electrolyte during charging. The decomposition of the electrolyte decreases the battery performance, especially the capacity, retention characteristics of the battery, cycle life characteristics, low temperature characteristics, and other similar properties. Therefore, it is important to restrain the decomposition reaction of the electrolyte with the surface of the negative electrode to improve the above-mentioned battery characteristics.
A great deal of research has been undertaken regarding solvents that can restrain the decomposition reaction of the electrolyte. When selecting a solvent, such factors as solubility of the electrolyte support salt, reactivity with the positive electrode, ion conductivity, cost, etc., are considered.
Non-aqueous solvents are regarded as preferable solvents that meet the above-mentioned criteria. The preferable non-aqueous solvents comprise ethylene carbonate, butylenes carbonate, dimethyl carbonate, methylethyl carbonate, diethyl carbonate, γ-butyrolactone, propionic acid methyl, propionic acid butyl, propionic acid ethyl, etc., and a mixture thereof.
In addition, several research projects dealing with restraining the decomposition reaction of the electrolyte by forming a film on the surface of the negative electrode in order to increase the battery performance are being undertaken.